1. Field of the Invention
The present invention relates to a color image display panel obtained by arraying multiple organic electroluminescence devices (organic EL devices) each having such a structure that a first electrode, a layer containing at least one kind of a low-molecular-weight luminous organic compound, and a second electrode are laminated in the stated order on a substrate and a method of producing the color image display panel, and a color image display apparatus using the color image display panel.
2. Description of the Related Art
A known organic EL device has such a structure that a first electrode formed on a substrate, an organic compound layer formed on the first electrode, and a second electrode formed on the organic compound layer are laminated.
FIGS. 11A and 11B are each schematic views illustrating an example of an image display panel on which organic EL devices are arrayed in a matrix fashion. FIG. 11A is a plan view illustrating the array of the respective pixels. FIG. 11B is a sectional view taken along the line 11B-11B of FIG. 11A. In each of FIGS. 11A and 11B, reference numeral 101 represents a substrate, 102 represents a first electrode, 103 represents an organic compound layer, and 104 represents a second electrode. In addition, reference numeral 105 represents a pixel specified by a region where the organic compound layer is interposed between the first electrode 102 and the second electrode 104.
Each of FIGS. 11A and 11B shows an example in which dot-shaped organic EL devices as pixels are arrayed on the surface of the substrate 101 in a matrix fashion, and each of the organic EL devices is formed so as to be capable of emitting light independently. The image display panel can display various images by appropriately selecting an organic EL device to be caused to emit light.
FIG. 12 shows an example of a sectional view of an image display panel having such a constitution that a device isolation film 110 is formed around each organic EL device 105 to prevent currents from crossing and flowing between adjacent devices. In FIG. 12, a protective layer 111 is further formed on the surface of each device, which prevent infiltration of water, air, or the like into a device portion from an environmental surrounding.
Examples of the shapes and array of multicolor sub-pixels when the sub-pixels are used as one group for forming a color image display panel will be described.
FIGS. 13A to 13C are each a plan view illustrating the array of sub-pixels. FIG. 13A out of FIGS. 13A to 13C is a schematic view illustrating the array of sub-pixels referred to as a stripe array. Devices having red, green, and blue luminescent colors are formed so as to be adjacent to one another because three primary colors are needed in a full-color image display panel. A group using the three adjacent devices as sub-pixels forms a pixel. When the colors of the sub-pixels are distinguished from one another in each of FIGS. 13A to 13C, red, green, and blue colors are represented by symbols R, G, and B, respectively. The same is applied to the following description.
Because a square pixel is typically used in the display of a computer, the sub-pixels are each formed into a rectangle having a long side-to-short side ratio of about 3, and sub-pixels adjacent to each other in the short side direction of the rectangle are defined as a pixel in many cases. Japanese Patent Application Laid-Open No. 2002-110345 refers to a method of producing a full-color organic EL image display panel having sub-pixels arrayed in a stripe fashion by using a stripe-shaped mask.
Even when sub-pixels having four or more kinds of luminescent colors (including a white color) are defined as one pixel for the purpose of, for example, obtaining brightness or extending the range of colors that can be displayed, a pixel is generally formed of a group of rectangular sub-pixels adjacent to each other in their short side directions. For example, even when there is no need for displaying full colors, and only two kinds of sub-pixels are used, a pixel is generally formed of a group of rectangular sub-pixels adjacent to each other in their short side directions.
FIG. 13B is a schematic view illustrating another array of sub-pixels referred to as an RGB delta array. The delta array is characterized in that a color array in an odd row deviates from that in an even row by a 1/2 pitch. An excellent resolution is achieved when a natural image is displayed on a portable, small display where the number of pixels is restricted.
The array of sub-pixels shown in FIG. 13C and referred to as a scramble array in which an array of sub-pixels in the upper row deviates from that in an n-th row by a 1/3 pitch is also similarly known.
Japanese Patent Application Laid-Open No. 2004-516630 proposes a display panel in which the positional constitution of the colors of sub-pixels in a pixel differs from that in an adjacent pixel with a view to improving the quality of a display image. In addition, the document discloses that when an electroluminescence material is deposited by an ink-jet printing step, the material is continuously deposited in a line shape slanted with respect to a pixel array, whereby the thickness of a deposited layer can be stabilized, and, furthermore, a display panel in which the positional constitution of the colors of sub-pixels in a pixel differs from that in any other pixel can be obtained.
FIG. 14 is a perspective view for describing a general step of producing a color image display panel by a vacuum deposition method through a mask. In FIG. 14, the same reference numerals as those of FIGS. 11A, 11B, 11C and 12 represent the same members as those of FIGS. 11A, 11B, 11C and 12, and reference numerals 201 and 202 represent a mask and a through-hole, respectively. In the example shown in the figure, the through-holes 202 of the mask 201 are each formed so as to coincide with the position of a sub-pixel having one specific luminescent color in the delta array.
An active matrix drive circuit (not shown) and the first electrode 102 connected to the circuit are formed on the substrate 101. The organic compound layer 103 capable of emitting red, green, or blue light is formed on the resultant through each through-hole 202 of the mask 201. Next, the mask 201 is shifted in a horizontal direction toward the position of an adjacent sub-pixel, and the organic compound layer 103 capable of emitting light having a luminescent color different from that of light emitted from the organic compound layer 103 previously formed is formed on the sub-pixel through the through-hole 202 of the mask 201. The foregoing operation is repeated three times, whereby the organic compound layers 103 corresponding to the respective red, green, and blue colors are each deposited at a predetermined position. Finally, the second electrode 104 is formed on the entirety of the resultant, and, as required, the protective layer 111 is further formed on the electrode, whereby the color image display panel is obtained.
FIG. 15 is a sectional view of the mask 201 taken along the line 15-15 of FIG. 14. The width of a sub-pixel is reduced so as to be comparable to the thickness of the mask 201 in a color image display panel having a high resolution. When a material emitter of such a large size that vacuum deposition can be rapidly performed is used, the thickness of the mask 201 must be reduced in order that the extent to which the thickness of a deposit at the center of the small through-hole 202 of the mask 201 differs from that at the peripheral portion of the through-hole may be alleviated. However, the mask 201 typically requires a thickness of at least about several tens of micrometers in order that the durability and handleability of the mask itself may be secured. In view of the foregoing, the following mask is used: a shaved portion 203 is formed so that each through-hole 202 expands on the material emitter side of the mask 201 as shown in FIG. 15, whereby the thickness of the peripheral portion of the through-hole 202 is reduced while the thickness of the mask 201 is increased so that the strength of the mask is secured.
When the array of rectangular sub-pixels is the above delta array or the above scramble array in a color image display panel on which organic EL devices having two or more kinds of different luminescent colors are arrayed, a vacuum deposition step involving the use of a mask involves the following problem.
FIGS. 16A and 16B are each a schematic view illustrating a mask for a vacuum deposition step for arraying sub-pixels in a delta array. FIG. 16A is a plan view illustrating the mask when viewed from its material emitter side, and FIG. 16B is a sectional view taken along the line 16B-16B of FIG. 16A. When each shaved portion 203 is formed so that each through-hole 202 expands toward the material emitter side as described above, the respective shaved portions 203 interfere with each other at a portion where the corners of the through-holes 202 are close to each other, whereby a portion where the thickness of the mask 201 is significantly small is formed.
As a result, the mask involves a problem in terms of strength: the mask is apt to split in a specific direction when an external force is applied to the mask. The production of a mask itself having an insufficient strength wastes a time period and a cost, and the number of times the mask can be repeatedly used in a vacuum deposition step reduces. In addition, the mask cannot be fixed to a jig by applying a sufficient tension, so the accuracy of position of the vacuum deposition step becomes low, and the yield in which a high-definition image display panel is produced reduces. In the case of the scramble array, a portion where the shaved portions 203 interfere with each other particularly strongly (not shown) is present, so the resultant mask is apt to tear particularly in an oblique direction, and it becomes additionally difficult to produce a mask that can be put into practical use.
The strength of the mask can be secured by reducing the size of each through-hole 202 so that the above interference does not occur. However, the area of each sub-pixel reduces, so the current density of a device must be increased in order that light with predetermined brightness may be emitted, and the increase adversely affects the luminous current efficiency and driving lifetime of an image display panel.
In short, a color image display panel which: is produced by employing a vacuum deposition step as described above; and has sub-pixels arrayed in a delta array or a scramble array involves problems to be solved in terms of, for example, yield, cost, performance, and lifetime. An image display apparatus using the image display panel as a component inherits the problems, so the apparatus involves problems in terms of, for example, cost, performance, and lifetime.
In other words, a conventional color image display panel having rectangular sub-pixels arrayed in a delta array or a scramble array involves the following problem: it is difficult to increase the opening ratio of each sub-pixel when the sub-pixel is produced by a vacuum deposition step involving the use of a mask. The problem is due to the presence of a portion where the corner portions of sub-pixels are close to each other. The problem described here is remarkable when a color image display panel is produced by a vacuum deposition step involving the use of a mask having a large opening ratio.